We Learned in Recent Years?
J. K
AUTZNER, H. M
LCOCHOVA, P. P
EICHLIntroduction
Although atrial fibrillation (AF) can be adequately controlled by drugs in the majority of patients, medicaments may fail in certain proportion of cases. In recent years, substantial progress has been made in both the elucidation of AF mechanisms and non-pharmacological treatment of this arrhythmia.
Catheter ablation is now considered to be a highly effective treatment option for the cure of symptomatic, drug-resistant AF [1–5]. Since 1998, when catheter ablation of focal sources of AF was first reported by Haïssaguerre et al. [1], several ablation techniques have been developed. Despite the diversity of individual ablation strategies, all of them have something in common: the ablation is performed either within or around the ostia of the pulmonary veins (PVs). Contrary to initial belief, pulmonary venous anatomy has been shown to be highly variable, and this increases the importance of imaging before or during the procedure. The aim of this review is to discuss what have we learned in recent years about the anatomy of the left atrium and PVs, and to illustrate the impact of this on the practice of catheter ablation.
Anatomical Studies
Probably the first mention of myocardial sleeves around pulmonary and caval veins originates from one of the Purkinje’s pupils, Ferdinandus Raeuschel, and dates back to 1836 [6]. In his medical dissertation, Raeuschel described muscular fibres on the surface of both caval veins and PVs up to
Department of Cardiology, Institute for Clinical and Experimental Medicine, Prague, Czech Republic
their branching to secondary vessels. Muscular sleeves around the PVs were later rediscovered in 1907 by Keith and Flack [7]. A detailed morphological description was published in the late 1960s [8]. However, the importance of PV anatomy went unrecognised until the era of catheter ablation for AF.
Reflecting the need of electrophysiologists for detailed knowledge of the pulmonary venous anatomy, Ho et al. carried out a thorough investigation of the arrangement and dimensions of the PVs in postmortem hearts [9].
Although some changes in the macroscopic anatomy and diameters were possible owing to fixation of the specimens, four distinct PV ostia were reported in 77% of cases and the rest presented with variant anatomy. In the second study by these authors [10], a common vestibule of the left PVs was described in three cases and a common orifice of the right PVs was found in a further two subjects (accounted for 25% incidence of variant PV anatomy).
Discounting the common ostia, the diameter of the PV ostia ranged from 8 mm to 21 mm (mean 12.5 mm). An identical proportion of common ostia of PVs (25%), usually on the left side of the left atrium, was reported in the study by Moubarak et al. [11]. Thus, the concept of four PVs with distinct ostia originating from the left atrium was debated for the first time in these early anatomical studies.
Pulmonary Venous Angiography
Pulmonary venous angiography was often used during the early period of our
experience with catheter ablation for AF. The main purpose was to determine
the position and size of the PV ostia and, additionally, to detect possible
stenosis of the veins after the procedure. Some of the studies revealed that
patients with AF have larger PV diameters in proportion to the enlarged left
atrium [12]. The average diameter, of superior PV ostia were 10.9–11.0 mm
in controls, and 13.1–13.6 mm in patients with AF. The corresponding
diameters of the inferior PVs were 7.5 mm and 8.3 mm, respectively. In the
light of contemporary experience, the above data suggest that the PV diam-
eters were measured within the tube-like portions of PVs and not at the
level of the actual PV–atrial junction. More realistic values of PV ostial
diameters (rang ing b etween 16.9 and 19.4 mm) were repor ted by
Vasamreddy et al. [13], who defined the PV ostium angiographically as the
junction of the PV with the left atrium and measured from both projections
with correction for a degree of magnification. The study revealed excellent
correlation between the angiographic measurements and MR angiographic
(MRA) measurements as analysed from 2D maximum intensity projections
and multiplanar reformations. Surprisingly, ostial diameters were found to
be similar in the two perpendicular planes, implying that the PV ostia are
circular in shape.
Wittkampf et al. [14] demonstrated for the first time how contrast angiography can be misleading in its representation of PVs. Comparing this method with 3D reconstruction of MRA images, they showed that the major- ity of the PV ostia are oval in shape, being longer in the superoinferior dimension than in the anteroposterior dimension. The mean ratios between maximum and minimum dimensions were 1.5 for left veins and 1.2 for right veins, which corresponds with a more circular shape of the right PVs.
Maximum diameters of PVs ranged between 15.9 and 18.7 mm, and left common PV ostium measured 27.3 mm on average.
The above experience suggests that neither PV angiography nor 2D view- ing format of MRA data provides an exact description of the true PV ostial shape. Despite this, many electrophysiologists still rely solely on PV angiog- raphy to guide catheter ablation of AF.
3D Imaging Techniques
The advent of modern 3D imaging techniques such as MRA and/or multide- tector CT angiography enabled detailed anatomical studies of PV anatomy and non-invasive assessment of PV stenosis after catheter ablation [15–19].
The PV ostia were found to be oval in shape with the anteroposterior dimen- sion less than the superoinferior dimension. Subjects with AF also presented with complex branching patterns, especially in the inferior PVs [16].
However, individual authors identified variant PV ostial anatomy in a vari- able proportion of subjects. For instance, the occurrence of a common left vestibule of PVs ranged from 3% to 32%. Even the largest study, by Mansour et al. [19], suggested that only 17% of cases presented with a common left trunk and 29% with additional right-sided PVs. Our experience suggests that the above inconsistency may arise from the fact that the majority of the studies used 2D formatting of data instead of true 3D reconstructions. Using special software for digital subtraction of arterial and venous phase and sub- sequent 3D reconstruction of the data (Fig. 1), we have demonstrated that the majority of patients (i.e. 75%) present with a common left vestibule [20].
Such a high occurrence of a common left vestibule suggests that this pattern should be defined as the ‘normal’ PV arrangement. In addition, comparing 2D and 3D formats we have shown that the true PV arrangement is often underestimated from 2D maximum intensity projections. Thus, only 3D reconstructions of the segmented MRA data can reveal the true anatomy of the left atrium, appendage, and PVs from different views.
A similar observation was made by Schwartzman et al. [21] using 3D
reconstructions of CT images, which they then correlated with those
obtained from intracardiac echocardiography. They revealed sligtly more
frequent occurrence of common vestibules on the left side and confirmed by
analysis of local electrograms the presence of typical fractionated or bifid potentials at these sites. Left atrial and PV dimensions were significantly greater in the AF group. However, after correcting for left atrial volume, all PV diameters were similar.
Thus, the use of 3D imaging techniques has changed our understanding of PV ostial anatomy significantly and has helped to identify PV vestibules that make up real PV–left atrial junctions. Their shape is predominantly oval and they tend to extend to various degrees into the posterior wall of the left atrium, especially on the left side. As a result, in many patients both left and right-sided vestibules are close to each other on the posterior wall. This may have important implications for the strategy of catheter ablation.
Fig. 1a–c.Different types of pulmonary vein branching patterns as assessed by 3D reconstructions of MRA imaging including virtual endoscopic images of the left pul- monary veins: a less common pattern of four separate ostia and early branching of the right superior vein, b prevailing pattern of short common vestibule or antrum of left-sided pulmonary veins, c long common left trunk and early branching of both right pulmonary veins. Au auricle, PV pulmonary veins
a
b
c
Intracardiac Echocardiography
Recently, studies have been published on the use of intracardiac echocardio- graphy (ICE) during catheter ablation [22–26]. The most widely used imag- ing modalities are either a mechanical system with rotating transducer obtaining images in 360° radial fashion (Boston Scientific, Natick, Mass., USA) or a phased-array system (Acuson, Mountain View, Calif., USA) with steerable 90° longitudinal imaging. The latter system offers a greater depth of penetration and the possibility of Doppler imaging including colour coding.
It has been reported that ICE can facilitate catheter ablation of AF by increasing efficacy and reducing complications [22]. The main advantages ICE offers are: (1) real-time delineation of cardiac anatomy, especially of PV ostia; (2) positioning of the catheter tip and assessment of its contact with the atrial wall; (3) visualisation of microbubble formation as a sign of tissue overheating; (4) assessment of PV flow and recognition of PV stenosis; and (5) early detection of thrombus and/or char formation. An ICE catheter posi- tioned in the middle of the right atrium provides clear images of the fossa ovalis and allows safe trans-septal puncture even in anticoagulated patients.
Given the high variability in PV arrangement, ICE provides very accurate information about the position of the catheter tip around the PV ostia, allowing precise positioning of the circular mapping catheter (Lasso, Biosense Webster, Diamond Bar, Calif., USA) at the exact level of the ostium.
It has been shown to visualise PV ostia better than angiography (Fig. 2), and
Fig. 2.Common vestibule of the left-sided pulmonary veins (doubleheaded arrows) as depicted by intracardiac ultrasound (ACUSON, Siemens, Mountain View, Calif., USA). LIPV left inferior pulmonary vein, LSPV left superior pulmonary vein
to ensure proper electrode alignment and contact [23]. Monitoring of microbubble formation can reveal tissue overheating [24–25] and thus min- imise the risk of thrombus formation and other complications such as atrio- oesophageal fistula. At the same time, ICE navigation minimises the risk of PV stenosis [26]. For attempts to use novel devices such as a balloon catheter for circumferential ultrasound ablation and/or a focused ultrasound balloon, ICE may provide an excellent tool for navigation within the PV ostia. On the basis of previous experience, ostial anatomy and resulting misalignment of the catheter were identified as the main reason for ineffective energy deliv- ery and failure of catheter ablation [27].
Implications for Catheter Ablation
All the above data suggest that the arrangement of the PV ostia is highly variable, and even results obtained by the same imaging technique may vary significantly. Pre-procedural 3D imaging appears to be the best tool to pro- vide an understanding of the anatomy and provides a basis for subsequent assessment of PV stenosis. True 3D reconstructions of images allow not only visualisation of supernumerary PVs but, especially, an appreciation of the morphology and size of PV ostia. This morphological knowledge may the modify strategy of catheter ablation in a given case. Repeat studies during follow-up can reveal PV stenosis. However, apart from PV angiography and/or fluoroscopy guidance around the circular mapping catheter in the PV, no on-line imaging was available until recently. Many electrophysiologists turned to an electroanatomical mapping system (CARTO, Biosense-Webster, Diamond Bar, Calif., USA) to reconstruct a virtual 3D anatomy of the left atrium and tag the position of the PV ostia as identified from PV angiogra- phy and/or from evaluation of catheter tip impedance during mapping. The advent of ICE provides real on-line control of the position of the catheter with respect to the PV ostia, and thus an additional potential benefit in catheter ablation of AF. There is mounting evidence that on-line imaging by means of ICE increases the success rate and minimises all potential compli- cations of the procedure. Besides the imaging itself, ICE allows titration of power delivery through monitoring for microbubbles as a sign of tissue overheating. It may also enable proper placement of novel devices such as the focused ultrasound catheter into the PV ostia.
Reflecting the need for intra-procedural navigation, various techniques of
image integration are being developed. One of them that is ready for clinical
use (CARTO Merge, Biosense-Webster, Diamond Bar, Calif., USA) integrates
pre-procedural 3D images (either 3D CT angiography or MRA) with the vir-
tual electroanatomical CARTO map constructed during the procedure. After
the initial process of registration, the imported 3D anatomical map of the left atrium and PVs could be used for catheter navigation. ICE guidance could be used in order to improve the registration process (Fig. 3).
According to our early experience with this software, reliable correlation between anatomical reconstructions and real-time CARTO maps of the left atrium and PVs can be obtained in a good proportion of patients. Image integration with other mapping modalities such as NAVx (St. Jude Medical, Minneapolis–St Paul, Minn., USA) is under development.
Conclusions
Lessons learned from sophisticated imaging techniques such as MRA or CT angiography suggest that the anatomy of the left atrium and PVs is very complex and highly variable. The most important discovery appears to be the fact that in a large proportion of patients left-sided PVs merge into a common vestibule or antrum. Recent evidence suggests that ablation at the level of this common antrum, if present, may have better efficacy and carry less risk of PV stenosis. This increases the need for either pre-procedural 3D imaging (and image integration) or intra-procedural imaging using ICE. The latter technology, especially, may become a universal guiding tool that allows precise catheter positioning, monitoring of energy delivery, and/or reduction of complications during these complex procedures.
Fig. 3.Computer tomography angiographic 3D image integrated with electroanatomi- cal mapping system (CARTO Merge, Biosense-Webster). After initial registration under intracardiac echocardiographic guidance, the anatomical image was fitted to an electroanatomical map and subsequently used to deploy ablation lines around pulmonary veins (black dots). A shows the left atrium in posterior view; B and C show virtual endoscopic images of the left- and right-sided pulmonary veins, respec- tively. LIPV left inferior pulmonary vein, LSPV left superior pulmonary vein, RIPV right inferior pulmonary vein, RSPV right superior pulmonary vein
References
1. Haïssaguerre M, Jais P, Shah DC et al (1998) Spontaneous initiation of atrial fibril- lation by ectopic beats originating in the pulmonary veins. N Engl J Med 98:1769–1775
2. Haïssaguerre M, Shah DC, Jais P et al (2000) Mapping-guided ablation of pul- monary veins to cure atrial fibrillation. Am J Cardiol 86(Suppl 1):K9-K19
3. Pappone C, Rosanio S, Oreto G et al (2000) Circumferential radiofrequency abla- tion of pulmonary vein ostia: a new anatomic approach for curing atrial fibrilla- tion. Circulation 102:2619–2628
4. Pappone C, Oreto G, Rosanio S et al (2001) Atrial electroanatomic remodeling after circumferential radiofrequency pulmonary vein ablation: efficacy of an anatomical approach in a large cohort of patients with atrial fibrillation. Circulation 104:2539–2544
5. Nademanee K, McKenzie J, Kosar E et al (2004) A new approach for catheter abla- tion of atrial fibrillation: mapping of the electrophysiologic substrate. J Am Coll Cardiol 43:2044–2053
6. Raeuschel F. De arteriarum et venarum structura. Dissertatio inauguralis anatomi- co-physiologica, Universitate Viadrina, Vratislaviae, 29 October 1836
7. Keith A, Flack M (1907) The form and the nature of muscular connections between the primary divisions of the vertebrate heart. J Anat Physiol 41:172 -189
8. Nathan H, Eliakin M (1966) The junction between the left atrium and the pul- monary veins. Circulation 34:412–422
9. Ho SY, Sanchez-Quintana D, Cabrera JA et al (1999) Anatomy of the left atrium:
implications for radiofrequency ablation of atrial fibrillation. J Cardiovasc Electrophysiol 10:1525–1533
10. Ho SY, Cabrera JA, Tran VH et al (2001) Architecture of the pulmonary veins: rele- vance to radiofrequency ablation. Heart 86:265–270
11. Moubarak JB, Rozwadowski JV, Strzalka CT et al (2000) Pulmonary veins – left atri- al junction: anatomic and histological study. Pacing Clin Electrophysiol 23:1836–1838
12. Lin WS, Prakash VS, Tai CT et al (2000) Pulmonary vein morphology in patients with paroxysmal atrial fibrillation initiated by ectopic beats originating from the pulmonary veins: implications for catheter ablation. Circulation 101:1274–1281 13. Vasamreddy CR, Jayam V, Lickfett L et al (2004) Technique and results of pul-
monary vein angiography in patients undergoing catheter ablation of atrial fibril- lation. J Cardiovasc Electrophysiol 15:21–26
14. Wittkampf FHM, Vonken EJ, Derksen R et al (2003) Pulmonary vein ostium geom- etry: analysis by magnetic resonance angiography. Circulation 107:21–23
15. Dill T, Neumann T, Ekinci O et al (2003) Pulmonary vein diameter reduction after radiofrequency catheter ablation for paroxysmal atrial fibrillation evaluated by contrast-enhanced three-dimensional magnetic resonance imaging. Circulation 107:845–850
16. Kato R, Lickfett L, Meininger G et al (2003) Pulmonary vein anatomy in patients undergoing catheter ablation of atrial fibrillation: lessons learned by use of mag- netic resonance imaging. Circulation 107:2004–2010
17. Takase B, Nagata M, Matsui T et al (2004) Pulmonary vein dimensions and varia- tion of branching pattern in patients with paroxysmal atrial fibrillation using mag- netic resonance angiography. Jpn Heart J 45:81–92
18. Scharf C, Sneider M, Case I et al (2003) Anatomy of the pulmonary veins in
patients with atrial fibrillation and effects of segmental ostial ablation analyzed by computed tomography. J Cardiovasc Electrophysiol 14:150–155
19. Mansour M, Holmvang G, Sosnovik D et al (2004) Assessment of pulmonary vein anatomic variability by magnetic resonance imaging: implications for catheter ablation techniques for atrial fibrillation. J Cardiovasc Electrophysiol 15:387–393 20. Mlcochova H, Cihak R, Tintera J et al (2004) Variability of pulmonary veins in
patients undergoing ablation of AF. Europace 6(1):80
21. Schwartzman D, Lacomis J, Wigginton WG (2003) Characterisation of left atrium and distal pulmonary vein morphology using multidimensional computed tomog- raphy. J Am Coll Cardiol 16:1349–1357
22. Cooper JM, Epstein LM (2001) Use of intracardiac echocardiography to guide abla- tion of atrial fibrillation. Circulation 104:3010–3013
23. Kalman JM, Fitzpatrick AP, Olgin JE et al (1997) Biophysical characteristics of radiofrequency lesion formation in vivo: dynamics of catheter tip-tissue contact evaluated by intracardiac echocardiography. Am Heart J 133:8–18
24. Marrouche N, Martin D, Wazni O et al (2003) Phased-array intracardiac echocar- diography monitoring during pulmonary vein isolation in patients with atrial fib- rillation: impact on outcome and complications. Circulation 107:2710–2716 25. Wood MA, Shaffer KM, Ellenbogen AL et al (2005) Microbubbles during radiofre-
quency ablation: composition and formation. Heart Rhythm 2:397–403
26. Saad EB, Rosillo A, Saad CP et al (2003) Pulmonary vein stenosis after radiofre- quency ablation of atrial fibrillation: functional characterisation, evolution, and influence of the ablation strategy. Circulation 108:3102–3107
27. Saliba W, Wilber D, Packer D et al (2002) Circumferential ultrasound ablation for pulmonary vein isolation: analysis of acute and chronic failures. J Cardiovasc Electrophysiol 13:957–961
